A study of the metabolism of the branched-chain amino acids has revealed a pathway of metabolism of leucine that is catabolic in bacteria and appears to be synthetic in humans. The pathway depends upon the activity of the enzyme, leucine 2, 3-aminomutase, an enzyme dependent upon adenosylcobalamin as a cofactor. Other enzymes which function in the pathway are Beta-leucine transaminase/deaminase, coenzyme A transferase, and thiolase. The relative carbon flux through this pathway and the pathway which is independent of cobalamin greatly favors the independent pathway in brain, heart, kidney, and liver. In the testis, however, the cobalamin-dependent pathway accounts for over forty percent of the carbon flux. This suggests that the metabolism of leucine may play an important role in this organ. The nature of the transaminase/deaminase will be examined and purification of the enzyme will be attempted. The relationship between enzyme activity and various disease states such as pernicious anemia and inborn errors of metabolism will be examined.